1. Field of the Invention
This invention relates to a process for treating waste paper and, more particularly, is concerned with a process for recovering a useful product from aqueous rejects which contain suspended solid material and which are derived from a plant for de-inking printed waste paper, de-inking plant waste being especially difficult to treat.
A process for converting waste paper into a pulp suitable for re-use will often include one or more operations designed, for example, to separate printing ink from the cellulosic fibres in the waste paper. Generally, in a plant for de-inking waste paper, the waste paper is first pulped, for example in a pulper provided with a low attrition rotor, in water containing various chemical reagents, at a consistency which is usually in the range from about 8% to about 18% by dry weight of waste paper. The chemical reagents used generally include sodium hydroxide which causes the fibers to swell and saponifies most commonly used printing ink vehicles, sodium silicate which acts as a pH buffer and agglomerates detached ink particles to a convenient size, and a surfactant which aids wetting and dispersion of the ink particles. The pulper is generally operated at a temperature in the range from 55.degree. C. to 70.degree. C. and at a pH in the range from 9 to 11. On completion of the pulping operation the pulp is usually diluted with water to about 4%-5% by weight of dry solids and then reconcentrated to a consistency of about 14%-16% by weight of dry solids: the dilution and subsequent reconcentration is advantageous because the dilute suspension is easier to pump from the pulper to the next stage, and because it makes it possible to return a proportion of the hot water and chemicals to the pulper.
The pulp suspension is then passed to a system of screens which remove relatively heavy foreign bodies, such as staples, paper clips and particles of grit, and also lightweight contaminants, such as plastics materials, and "stickies" which arise from adhesives used in paper coating, binding or laminating, and from hydrophobic reagents, such as sizing agents.
The suspension passing through the screens is then fed to a treatment plant which usually comprises one or more froth flotation cells, or one or more washing units, or a combination of flotation cells and washing units.
For example, a typical treatment might comprise one or more froth flotation cells, each of which is provided with an impeller and means to introduce air in the form of fine bubbles in the vicinity of the impeller. An additional chemical, known as a collector, is added to the pulp suspension prior to the flotation operation. The collector may be, for example, a fatty acid soap or a non-ionic surfactant such as an ethylene oxide/propylene oxide copolymer. The collector attaches itself preferentially to the ink particles, rendering them hydrophobic, so that they have greater affinity for the air bubbles than for the water, and are lifted to the surface by the bubbles as they rise through the suspension. It may be necessary, in order to provide a sufficiency stable foam, to add also a water-soluble calcium salt to ensure that the water has a hardness of at least 200 mg of CaCO.sub.3. Ink particles having sizes in the range from about 20 .mu.m to about 200 .mu.m are concentrated in the froth which overflows from the top of the flotation cells, while a pulp suspension depleted in ink is withdrawn from the bottom. The pulp suspension from the bottom of the flotation cells is then washed with water, and subsequently dewatered, to remove as much as possible of the remaining ink residue. The dewatered pulp is then generally dispersed with a surfactant, which may be, for example, of the nonylphenol ethoxylate or ethoxylated alcohol type. The dispersion is effected by treating the mixture of pulp and dispersing agent with steam to raise its temperature to a value within the range 50-90.degree. C., and subjecting the hot mixture to intense mechanical agitation whereby remaining ink particles are generally reduced in size to below about 20 m, at which size they remain in suspension and become invisible to the naked eye. The pulp containing finely dispersed ink particles is then passed through a fine slotted screen to remove any agglomerates or remaining sticky or resinous material and finally dewatered to produce a substantially de-inked pulp. The water recovered from the dewatering operations is generally cloudy as a result of its content of fine particulate material which may comprise, for example, inorganic filler particles or organic particles such as ink or fine fibres. It is necessary to remove as much as possible of this particulate material before this water is suitable for recycling for re-use in earlier stages of the de-inking process. The cloudy water may be cleaned, for example, in microflotation cells in which fine bubbles of air are injected at the base of the cells and the particulate material is carried upwards by air bubbles which attach themselves thereto.
The treatment plant reject, which is usually in the form of a suspension, or slurry, comprising the froth product from a flotation stage or the suspension that has passed through the screen of a washing stage generally contains, in addition to the ink particles, a substantial proportion of the cellulose fibers and inorganic filler (pigment) particles which were originally present in the waste paper. These filler particles generally consist predominantly of a mixture of kaolin clay and calcium carbonate in various proportions, although other inorganic filler particles such as talc, calcium sulphate or titanium dioxide may also be present in minor proportions.
The reject or effluent stream from a waste paper de-inking plant is usually known as a `sludge`. Such a sludge includes, in addition to the insoluble, potentially useful particulate and fibrous solid material, inking materials such as carbon and various organic inks, other organic substances such as latices and other adhesives, sizing agents and the like. Such a sludge is generally a sticky, highly discolored or gray mass. Furthermore, the water of the sludge contains a variety of soluble substances which reduce the usability of the water.
Thus, the useful inorganic particulate materials contained in the sludge from a de-inking plant are highly discolored and difficult to separate, difficult to dewater and are associated with water undesirably containing a high level of dissolved species. In consequence, a sludge from a de-inking sludge is usually regarded as an unusable waste and in the prior art has generally been discarded or incinerated to reduce the volume of waste, the ash then being discarded.
Hitherto, there has been, therefore, a loss to the paper maker of potentially useful inorganic particulate materials. Also, the paper maker has had to meet the expense of disposing of the treatment plant reject in a manner which has no undesirable effects on the environment.
2. Summary of the prior art
U.S. Pat. No. 4,115,188 describes a process for chemically treating waste water from a paper mill. In the disclosure in this prior patent it is stated that the waste water treated may be "from the various pulpings, refinings, deinkings, cleanings, dilutions, etc, involved in either a primary or secondary paper mill paper-making process". The implication is that it is not the primary effluent from a de-inking plant which is treated but rather a dilute stream in which waste from a de-inking plant might be only one of various constituents. The darkness or composition of the waste water treated is not discussed. In the described process the waste water is treated by adding an acid, preferably sulphuric acid, and then adding an alkali, preferably calcium hydroxide. The changes in pH produced cause the solids suspended in the waste water to be flocculated which allows them to be more easily separated eg by filtration, decantation, sedimentation etc thereby allowing the waste water to be clarified. The patent description contemplates the possibility of utilizing the separated solids in a number of ways, including providing a pulp source for low grade paper which may be re-used in the paper making process. The patent description contemplates the formation of some calcium sulphate as "ash" as part of the separated solids. However, such formation appears not to be deliberate but rather to be incidental to the flocculation effect caused by pH changes. In any case, in the preferred embodiment of the patent description 320 parts per million H.sub.2 SO.sub.4 and 250 parts per million slaked lime are added which will produce CaSO.sub.4 in a concentration of about 500 parts per million. CaSO.sub.4 has an appreciable solubility in water, viz about 1600 parts per million at room temperature. Therefore all CaSO.sub.4 produced as in the preferred embodiment of the prior art described in U.S. Pat. No. 4,115,188 is likely to dissolve rather than precipitate. The amount of added acid and alkali in the preferred embodiment suggest that the solids concentration of the treated waste water is about 0.1% to 0.2% by weight.
U.S. Pat. No. 3,354,028 describes a process for clarifying effluent derived from a process for de-inking printed paper. The objective of the process, is clearly to recover in a clean and reusable state the water which has been used in the process for de-inking the printed waste paper. It is noted in the reference that the liquid effluent from a de-inking plant typically includes a suspension of fine particulate materials in water. These particulate materials may be colloidal or non-colloidal, and may according to the reference comprise ink (both ink vehicle and pigment), paper fibres, paper fillers, dust, dirt and other waste solid and liquid material. This reference is principally concerned with the colloidal material, ie that having particles in the size range from 1 nm to 100 nm. The liquid effluent generally has a high turbidity and an object of the process described is to reduce this turbidity. The process described in this reference comprises chemically reversing the effluent pH, ie by raising the pH, and then lowering it, or vice versa. The pH is preferably raised with the aid of an alkali metal or alkaline earth metal hypochlorite, especially sodium, potassium or calcium hypochlorite. The result of this treatment is to flocculate the suspended fine material in the effluent suspension to give a good separation between solid matter and clear water. The flocculated solid matter may be separated from the water either by gravitational sedimentation or by froth flotation. The de-inking process generally precedes the step of flocculating the fine solid material in the effluent suspension. The process described in this reference is concerned solely with recovering the water from an aqueous effluent suspension from a printed waste paper de-inking process in a clean and reusable form. It is almost totally silent about what happens to the flocculated fine solid material after it has been separated from the clarified water. This specification gives no help at all to the skilled reader as to how he might recover and reuse the flocculated solid material for example in a paper making process.